Well it depends on where and you taxi
Rick6a said:
Rudi,
The instructions included with my 6A's builders manual says that the shimmy phenomena is a "rather nebulous thing." Some airplanes experience it, other's don't. Now I have this theory......but it is only a theory that may account for shimmy problems in some airplanes.
Rudi I agree NOT every gear will shimmy, but I think it's more complicated. It has to do with everything, tire temp/pressure/alignment, taxi surface and the natural frequency of the spring (gear leg). Its the interaction of the tire, surface it is rolling on and the basic RV gear desgin that makes the wiggle. On some surfaces, at the right speed, you will shimmy almost any gear leg.
Engineers have all the math to describe what is happening. It does not mean Van did a bad job. There is always a compromise between other design goals (low weight, drag, cost) and making the gear Heck for Stout.
With a tapered steel rod with a tire cantilevered of the end at odd angles, the gear will bend and twist. Once it moves it "springs back". If the speed and surface are producing the right kind of resistance at the gears natural frequency you will get the shimmy.
The focus on the tire being balanced is right on. That is job #1. Any thing that is a "forcing function" or produces vibratory excitation of the spring gear will aggravate a gear leg/wheel shimmy.
It also depends on
how you operate (taxi speed) and
where you operated (surface, temp?).
I think
temp has to do more with the damping and friction of the tire than the affect on steel. The rubber in the tire when cold or hot changes.
A surface like asphalt can change with temp. I remember sitting in the line-up at Laguardia in the summer and just sinking into the asphalt. To get moving again I needed LOTS of power to get out of the hole the 737 made sitting too long in one spot. B737 may weigh less than 150k-lbs, but they only have two wheel mains, unlike other planes with 4 or more wheel mains. You could see all the dimples in the ground where planes where stopped. Same at a little airport, the taxi or runway surface waves and dips vary with temp but may be too small to see but affects you flexible RV gear. I notice it in my car or motorcycles different rain groves in the road interact with my particular tread pattern differently.
Tire alignment as someone mentioned this adding to the issue of gear shimmy. Toe in/out is no doubt is part of it. We have ZERO adjustment unless once installed, unless you permanently bend/twist the gear past the elastic region. Once you drill (old days) or locate the firewall and engine mount (or gear towers RV-8 and RV-A's) your gear geometry is set. If one tire is trying to track straight and the other wants to wonder outboard,
it will make a series of little swerves as it tracks out and than springs back. Given the right speed and surface that gear leg will vibrate and the vibrations can get larger and larger if exited near natural frequency of the WHOLE SYSTEM. The "system" is the tire ground interface, tire, axle, gear fairing and steel gear leg it self. They all work together and the interaction is complicated.
If you want to lower your chance you "dampen" the gear leg so even if exited it's deflections are smaller or they dampen out. A wood stiffener may also increase its small deflection stiffness which changes it's natural frequency, but I think the main purpose is dampener of shock absorber for the gears bending and twisting. It will still bend and twist but it will not spring back as far.
Shimmy is not too hazardous. It does put wear and tear on the brake lines and wheel fairing.
With that said, I don't think the plans made stiffeners will make you 100% immune to shimmy either. If the conditions are right, like very high speed taxi on a semi-rough surface, tire pressure/temp/alignment are right, you will shimmy. When I say rough, I mean it could be small surface irregularities that are just at the right size and shape to excite the gear wiggle. Even a smooth surface may produce a shimmy if the tire pressure/balance/alignment is not correct (I think).
Therefore go with out and install later if needed is my advice. Look it is the nature of the beast. Long tapered steel rods at angles with a cantilever wheels will always act up time to time. In the Pro column the gear is light, low drag, cost effective, easy to build and works well overall. In fact its nature to take abuse and protect the main airframe his a good thing for some pilots who have dropped their bird in.
